platform/x86: int3472: make common part a separate module

platform/x86: int3472: make common part a separate module

Linking an object file into multiple modules is not supported
and causes a W=1 warning:

scripts/Makefile.build:236: drivers/platform/x86/intel/int3472/Makefile: common.o is added to multiple modules: intel_skl_int3472_discrete intel_skl_int3472_tps68470

Split out the common part here into a separate module to make it
more reliable.

Fixes: a2f9fbc247ee ("platform/x86: int3472: Split into 2 drivers")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240529095009.1895618-1-arnd@kernel.org
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>